U.S. patent number 4,932,034 [Application Number 07/414,585] was granted by the patent office on 1990-06-05 for distributed feedback semiconductor laser device and current injection.
This patent grant is currently assigned to Kokusai Denshin Denwa Kabushiki Kaisha. Invention is credited to Shigeyuki Akiba, Yuichi Matsushima, Masashi Usami.
United States Patent |
4,932,034 |
Usami , et al. |
June 5, 1990 |
Distributed feedback semiconductor laser device and current
injection
Abstract
A distributed feedback semiconductor device is disclosed which
has a diffraction grating disposed near a light emitting active
layer, a double hetero structure with the active layer sandwiched
between n- and p-type semiconductors and n- and p-side electrodes
for injection a current into the active layer, one of the n- and
p-side electrodes being divided into a plurality of electrodes, and
in which a current is injected into the active layer across the n-
and p-side electrodes for laser oscillation to obtain output light.
A first current source is connected to each of electrodes into
which one of the n- and p-side electrodes is divided, and a second
current source is connected to the divided electrodes via
resistors, for injecting a current into the active layer in a
desired ratio. The first and second current sources are controlled
in accordance with the light emitting state of the active layer. In
operation, a current is injected into the active layer through the
divided electrodes while controlling the injected-current density
in the active layer to be uniform in the direction of travel of
light until the injected current reaches a threshold current at
which the distributed feedback semiconductor laser device starts to
oscillate, and a current thereafter injected while controlling the
injected-current density to be maximum in at least that region of
the active layer in which the light intensity is maximum in the
direction of travel of light.
Inventors: |
Usami; Masashi (Tokyo,
JP), Akiba; Shigeyuki (Tokyo, JP),
Matsushima; Yuichi (Tanashi, JP) |
Assignee: |
Kokusai Denshin Denwa Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
13021734 |
Appl.
No.: |
07/414,585 |
Filed: |
September 27, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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314483 |
Feb 23, 1989 |
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Foreign Application Priority Data
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Mar 11, 1988 [JP] |
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63-56246 |
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Current U.S.
Class: |
372/96; 372/32;
372/38.01; 372/46.01; 372/50.11 |
Current CPC
Class: |
H01S
5/0625 (20130101); H01S 5/0683 (20130101); H01S
5/164 (20130101); H01S 5/06255 (20130101); H01S
5/06258 (20130101); H01S 5/06832 (20130101); H01S
5/12 (20130101); H01S 5/124 (20130101) |
Current International
Class: |
H01S
5/00 (20060101); H01S 5/16 (20060101); H01S
5/0683 (20060101); H01S 5/0625 (20060101); H01S
5/12 (20060101); H01S 003/08 (); H01S 003/19 () |
Field of
Search: |
;372/96,44,38,32,46,29,108 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Sikes; William L.
Assistant Examiner: Epps; Georgia Y.
Attorney, Agent or Firm: Lobato; Emmanuel J. Burns; Robert
E.
Parent Case Text
This is a continuation of application Ser. No. 07/314,483, filed
Feb. 23, 1989 now abandoned.
Claims
What we claim is:
1. A distributed feedback semiconductor laser device which is
provided with a diffraction grating provided near a light emitting
active layer, a double hetero structure with the active layer
sandwiched between n- and p-type semiconductors, and n- and p-side
electrodes for injecting a current into the active layer,
characterized by the provision of:
a first current source connected to each of electrodes into which
one of the n- and p-side electrodes is divided; and
a second current source connected to the divided electrodes via
resistors, for injecting a current into the active layer in a
desired ratio;
the first and second current sources being controlled in accordance
with the light emitting state of the active layer.
2. A distributed feedback semiconductor laser device according to
claim 1, characterized in that the first current source injects a
current into the active layer uniformly via the divided electrodes
and the second current source injects a current into the active
layer via the divided electrodes so that the current density is
maximum in that region of the active layer where the light
intensity is higher than in the other regions.
3. A distributed feedback semiconductor laser device according to
claim 1, characterized in that one of the divided electrodes is
disposed on that region of the active layer where the diffraction
grating has a phase shift point.
4. A distributed feedback semiconductor laser device according to
claim 3, characterized in that the current density in the region
where the diffraction grating has the phase shift point becomes
higher than in the other regions of the active layer.
5. A distributed feedback semiconductor laser device according to
claim 1, characterized in that the one of the divided electrodes is
disposed in the center of the active layer.
6. A current injection method for a distributed feedback
semiconductor laser device which has a diffraction grating disposed
near a light emitting active layer, a double hetero structure with
the active layer sandwiched between n- and p-type semiconductors
and n- and p-side electrodes for injection a current into the
active layer, one of the n- and p-side electrodes being divided
into a plurality of electrodes, and in which a current is injected
into the active layer across the n- and p-side electrodes for laser
oscillation to obtain output light, characterized by the inclusion
of:
a first step of injecting a current into the active layer through
the divided electrodes while controlling the injected-current
density in the active layer to be uniform in the direction of
travel of light until the injected current reaches a threshold
current at which the distributed feedback semiconductor laser
device starts to oscillate; and
a second step of injecting a current thereafter while controlling
the injected-current density to be maximum in at least that region
of the active layer in which the light intensity is maximum in the
direction of travel of light.
7. A distributed feedback semiconductor laser device comprising: a
diffraction grating provided near a light emitting active layer, a
double hetero structure with the active layer sanwiched between an
n-type semiconductor and a p-type semiconductor, and an n-side
electrode and a p-side electrode one of which is divided into
several electrodes for injecting a current into the active
layer,
characterized by the provision of:
a first current source connected to each of said several divided
electrodes into which one of the n-side electrode and the p-side
electrode is divided;
a plurality of resistors equal in number to said several divided
electrodes and connected thereto;
a second current source connected to the divided electrodes via
corresponding resistors, for injecting a current into the active
layer at a desired ratio in current density for each of the divided
electrodes; and
control means comprising a photodiode disposed for absorbing a
backward output light from the active layer to produce output light
employed for controlling the first current source and the second
current source in accordance with the light emitting state of the
active layer.
8. A distributed feedback semiconductor laser device according to
claim 7, characterized in that the first current source is
connected directly to said divided electrodes and injects a current
into the active layer uniformly via the divided electrodes, and the
second current source injects a current into the active layer via
the divided electrodes so that the current density is maximum in
that region of the active layer where the light intensity is higher
than in the other regions.
9. A distributed feedback semiconductor laser device according to
claim 7, characterized in that a center one of the divided
electrodes is disposed overlying a region of the active layer where
the diffraction grating has a phase shift point.
10. A distributed feedback semiconductor laser device according to
claim 9, characterized in that the current density in the region
where the diffraction grating has the phase shift point is higher
than in other regions of the active layer.
11. A distributed feedback semiconductor laser device according to
claim 7, characterized in that said one of the divided electrodes
overlies the center of the active layer.
12. A current injection method for a distributed feedback
semiconductor laser during which has a diffraction grating disposed
near a light emitting active layer, a double hetero structure with
the active layer sandwiched between n- and p-type semiconductors
and n- and p-side electrodes for injecting a current into the
active layer, one of the n- and p-side electrodes being divided
into a plurality of electrodes, and in which a current is injected
into the active layer across the n- and p-side electrodes for laser
oscillation to obtain output light, characterized by including
of:
a first step of injecting a current into the active layer through
the divided electrodes while controlling the injected-current
density in the active layer to be uniform in the direction of
travel of light until the injected current reaches a threshold
current at which the distributed feedback semiconductor laser
device starts to oscillate; and
a second step of injecting a current into the active layer through
the divided electrodes thereafter while controlling the
injected-current density to maximize it in that region of the
active layer in which the light intensity is maximum in the
direction of travel of light.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a distributed feedback
semiconductor laser having a diffraction grating and a current
injection method therefor.
In an ordinary semiconductor laser, light intensity in its active
layer during oscillation is distributed nonuniformly in the
direction of travel of light (hereinafter referred to as the "axial
direction"). The nonuniformity of light intensity in the axial
direction is particularly remarkable in a phase shifted distributed
feedback semiconductor laser (hereinafter referred to as the "phase
shifted DFB laser") which has incorporated therein a phase shifted
diffraction grating of excellent wavelength selectively. It has
been reported that, in the case of such a nonuniform distribution
of light intensity, if the injected current density distribution is
uniform, the refractive index varies owing to spatial hole burning
in the axial direction, by which a threshold gain difference
between a main mode and a sub-mode (hereinafter referred to as the
"oscillation threshold value gain difference"), which is an index
of a single-wavelength operation, varies with an increase in the
amount of current injected (see Soda et al., IEEE J. Quantum
Electron., Vol. QE-23, pp 804-814, 1987.) That is to say, the
higher the light intensity, the more carriers are consumed by
stimulated emission, and consequently, the carrier density in that
region decreases relative to the light intensity, with the result
that the carrier density has, in the axial direction, a nonuniform
distribution reverse from the light intensity distribution. On the
other hand, the refractive index of semiconductor varies with the
internal carrier density and decreases (or increases) when the
carrier density becomes high (or low). Therefore the refractive
index also has a distribution depending on the nonuniform carrier
density distribution. It has been observed not only that this
refractive index variation decreases the threshold gain difference,
which leads to degradation of the single-wavelength operation, but
also that the carrier density variation makes the laser output
light readily saturable during a high output operation of the
laser.
The present inventors have filed a patent application on a light
semiconductor device of a structure in which its electric
resistance ratio is distributed nonuniformly and the injected
current density is distributed substantially in proportion to the
light intensity distribution in the axial direction so as to avoid
the above-mentioned problems (Japan. patent application No.
168314/87). This is intended to obtain substantially uniform net
carrier density distribution and refractive index distribution by
providing an injected current density distribution substantially
opposite in direction from the carrier density distribution made
nonuniform by spatial hole burning in the axial direction. With
such a structure, it is possible to decrease the refractive index
variation by spatial hole burning in the axial direction which
increases with the current being injected. However, no matter what
electric resistance ratio distribution (or injection current
density distribution) may be used, a change in the injected current
inevitably causes some variations in the refractive index in the
axial direction, and consequently, the above-mentioned structure
cannot be applied to the case where the dynamic range of current is
selected so large.
According to the above method, the injected current density
distribution is fixed independently of the current being injected,
and consequently, the distribution remains unchanged even during
the injection of current below the threshold current. On the other
hand, the refractive index variation by hole burning does not occur
until oscillation; so that when the injected current is smaller
than the threshold current, the refractive index will be changed by
the nonuniform injected current density distribution alone. Because
of this refractive index variation, the injection of current of an
arbitrary value greater than the threshold current does not provide
an injected current density distribution which always completely
cancels the refractive index variation by hole burning.
As described above, the conventional semiconductor laser device
utilizing nonuniform current injection and the current injection
method therefor cannot completely suppress the refractive index
variation by spatial hole burning in the axial direction, and hence
incur deterioration of the single-wavelength selectively and
liability to saturation of the laser output light at the time of
high current injection, leading to the problem that no stable laser
output can be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a distributed
feedback laser device and its current injection method which
overcome the above-mentioned defects of the prior art and ensure
the stable laser output.
According to an aspect of the present invention, in a distributed
feedback semiconductor laser device which has a diffraction grating
disposed near a light emitting active layer, a double hetero
structure with the active layer sandwiching between n- and p-type
semiconductors, and n- and p-side electrodes for injecting a
current into the active layer, one of the n- and p-side electrodes
is divided into a plurality of electrodes, a first current source
is connected to the divided electrodes, a second current source is
also connected to the divided electrodes but via resistors for
injecting a current into the active layer at a desired current
ratio, and the first and second current sources are controlled in
accordance with the state of light emission of the active
layer.
According to another aspect of the present invention, in a current
injection method for a distributed feedback, semiconductor laser
device which has a diffraction grating disposed near a light
emitting active layer, a double hetero structure with the active
layer sandwiched between n- and p-type semiconductors, and n- and
p-side electrodes for injecting current into the active layer, one
of the n- and p-side electrodes being divided into a plurality of
electrodes, and in which a current is injected into the active
layer across the n- and p-side electrodes to oscillate the laser
device to obtain therefrom output light, there are included a first
step of injecting a current into the active layer through the
divided electrodes while controlling the injected current density
in the active layer to be uniform in the axial direction until the
injected current reaches a threshold current at which the laser
device starts to oscillate, and a second step of injecting a
current thereafter while controlling the injected current density
to be maximum in at least that region of the active layer in which
the light intensity is maximum in the axial direction.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described in detail below in
comparison with prior art with reference to the accompanying
drawings, in which:
FIG. 1 is a graph showing the results of calculation of the
injection current dependence of the threshold gain difference of a
conventional quarter-wave phase shifted DFB laser;
FIGS. 2A, 2B and 2C are a block diagram illustrating the
construction of the quarter-wave phase shifted DFB laser of the
present invention, a graph showing its light intensity distribution
and current density distribution, and a graph showing its injection
current dependence of the threshold gain difference,
respectively;
FIG. 3 is a graph showing laser current-laser output
characteristics of this invention laser and the conventional one;
and
FIGS. 4A and 4B are block diagram illustrating a second embodiment
of the DFB laser of the present invention and a graph showing its
light intensity distribution and current density distribution.
DETAILED DESCRIPTION
To make differences between prior art and the present invention
clear, characteristics of prior art will first be described.
FIG. 1 shows the injected current dependence of the threshold value
gain difference in the cases of a uniform current injection into a
conventional quarter-wave shifted DFB laser (indicated by the solid
line) and a current injection thereinto with a distribution
proportional to the light intensity distribution (indicated by the
broken line). It appears from FIG. 1 that the uniform current
injection causes a monotonous decrease in the threshold current
gain difference with an increase in the current and that control of
the injected current density makes it possible to maintain a large
threshold gain difference until high current injection as compared
with the case of not effecting such control, whereby the refractive
index variation by hole burning is suppressed to some extent. In
this instance, the injected current of a maximum value (M) is
present in the threshold gain difference, and at this current
value, the refractive index variations by hole burning and
nonuniform current injection are completely cancelled each other,
but in a lower current region the effect by the nonuniform current
injection is predominant and in a higher current region the effect
by the hole burning is predominant. On this account, the threshold
gain difference decreases not only at the threshold current but
also at the high current injection. While the above has shown a
case where the injected current density distribution is in
proportion to the light intensity distribution, the above-mentioned
tendency will still persist even if the degree of nonuniformity of
the current injection is changed or other similar distributions are
employed.
With reference to the accompanying drawings, the present invention
will hereinafter be described in detail. In the following, the same
parts will be identified by the same reference numerals and no
duplicate description will be given thereof.
(EMBODIMENT 1)
FIG. 2A illustrates the construction of an asymmetric quarter-wave
shifted DFB laser device according to a first embodiment of the
present invention. The asymmetric quarter-wave shifted DFB laser of
this embodiment has a structure in which an n-type InGaAsP
waveguide layer 2, an InGaAsP active layer 3, a p-type InGaAsP
buffer layer 4 and p-type InP clad layer 5 are laminated on an
n-type InP substrate 1 and a laser region 30 is formed by a
diffraction grating 8 which has a quarter-wave phase shift point 9
provided by periodically changing the film thickness of the n-type
InGaAsP waveguide layer 2 and causes an effective periodic
refractive index variation accordingly. Referance numeral 100
indicates forward laser output light (hereinafter referred to
simply as the "forward output light") and 101 backward laser output
light (hereinafter referred to simply as the "backward output
light"). The quarter-wave phase shift point 9 is located a little
to the side of the forward output light 100 relative to the center
of the laser region 30 so that the forward output light 100 becomes
larger than the backward output light 101. The laser region 30 is
divided into a region B near the quarter-wave phase shift point 9
and regions A and C near opposite end faces of the laser, and their
lengths are selected in the ratio of A:B:C=2:4:4, for example. On
the other hand, the active layer 3 is closed at its both ends with
the p-type InP clad layer 5 and a p-type layer 6 of a greater
energy gap than that of the active layer 6, forming window regions
31. Further, the overlapping portions of the laser region 30 and
the window regions 31 are each overlaid with a p-type InGaAsP cap
layer 7 for ohmic contact with an electrode. Reference numerals 20,
21 and 22 indicate p-side electrodes made of Au/Cr, and these
electrodes correspond to the regions A, B and C, respectively.
Reference numeral 23 denotes an n-side electrode made of Au/AuSn,
10 zinc diffused regions for reducing the contact resistance with
the electrodes, 50 and 51 current sources for laser driving use,
and 52, 53 and 54 resistors for injecting currents into the
respective regions in a desired ratio (J.sub.2A :J.sub.2B
:J.sub.2C). A portion of the backward output light 101 is absorbed
by a photodiode 56, which has its output connected to a current
controller 55 which controls the current sources 50 and 51.
Table 1 shows the relationships between the current sources 50 and
51 and the photodiode 56. In connection with this embodiment, a
description will be given of a current injection method which
gradually increases the injection current from zero until the DFB
laser oscillates.
(1) When the DFB laser is not in oscillation, that is, when the
photodiode 56 yields no outputs, the current controller 55 controls
the current source 51 not to produce its output and the current
source 50 to yield a gradually increasing an injection current.
(2) Next, the output of the current source 50 is fixed by the
current controller 55 at a time point when the DFB laser starts to
oscillate, that is, at the instant when the protodiode 56 yields
the output.
(3) The current, which is injected for raising the level of the
output light created by the oscillation of the DFB laser to obtain
a stable output light, is controlled by the current source 51
having connected thereto the resistors 52, 53 and 54 of
predetermined resistance values so that the afore-mentioned
injected current density becomes maximum in that region of the
active layer where the light intensity is maximum in the axial
direction.
By the above three steps, when the injection current is below the
threshold current, the current is injected into the respective
regions with a uniform current density, and when the injection
current is above the threshold current, the current exceeding the
threshold value current is injected with a current density
distribution corresponding to the light intensity distribution
through control of the values of the resistors 52, 53 and 54.
TABLE 1 ______________________________________ Photodiode output
Current source 50 Current source 51
______________________________________ No Controlled J.sub.2 = 0
Yes Fixed to a value Controlled at the time of threshold current
______________________________________
FIG. 2B is a graph showing the light intensity distribution and the
injected current density distribution characteristic of the present
invention in the axial direction in the active layer 3 in FIG. 2A.
The light intensity distribution is a nonuniform one in which the
light intensity is maximum at the quarter-wave phase shift point.
The current above the threshold current can be distributed in
agreement with this light intensity distribution by injecting the
current into the respective regions in a current density ratio of
J.sub.2A :J.sub.2B :J.sub.2C =2:3:1, for instance.
FIG. 2C shows the relationship, obtained by calculation, between
the threshold gain difference and the injected current at the time
of current injection with the distribution depicted in FIG. 2B. As
will be seen from FIG. 2C, the threshold value gain difference
remains substantially constant at a large value regardless of the
current injected, indicating substantially complete suppression of
the spatial hole burning in the axial direction. Thus, the present
invention is supported theoretically as well. Further, the number
of division of the p-side electrode is required to be at least
three in the case of the quarter-wave phase shifted DFB laser;
namely, the p-side electrode needs only to be divided into a
plurality of electrodes so that they lie on the right side and the
left side of the electrode 21 overlying the phase shift portion of
the diffraction grating (i.e. in the directions of travel of
light).
FIG. 3 shows the relationships between the forward output current
100 and the injection current in the same device measured in the
cases where a current was injected uniformly (indicated by the
broken line) and where the current was injected with a current
density distribution of J.sub.2A :J.sub.2B :J.sub.2C =2:3:1 as in
the above-described embodiment (indicated by the solid line). It
has been ascertained that the present invention improves the
differential quantum efficiency and maximum output of the forward
output light and ensures oscillation of almost all devices at
single wavelengths.
(EMBODIMENT 2)
FIG. 4A illustrates a second embodiment of the present invention.
This embodiment differs from the first embodiment in that a
diffraction grating 11 has no quarter-wave phase shift portion,
that no window region is provided at either end of the active layer
3, that the end facet from which the forward output light 100 is
emitted is coated with a non-reflective coating film 24, that the
end facet from which the backward output light 101 is emitted is
coated with a lightly reflective coating film 25, and that the
p-side electrode and the laser region 30 are both divided into two.
That is, when no phase shift portion is provided, at least one
electrode needs only to be provided at the side of each end
facet.
Sometimes such a DFB laser does not oscillate at a single
wavelength according to the phase of the diffraction grating at
either end facet. However, this laser is effective for use in
obtaining a high forward output light, because the ratio of the
forward output light 100 to the backward output light 101 can be
selected large.
FIG. 4B is a graph showing, by way of example, the light intensity
distribution in the axial direction (indicated by the full line)
and the injected current density distribution (indicated by the
broken line) characteristics of the present invention, in the
active layer 3 of the laser shown in FIG. 4A. When the current
density ratio for current exceeding the threshold value is selected
so that, for example, J.sub.2A :J.sub.2B =1:2, the actual carrier
density distribution can be made substantially uniform and no
spatial hole burning in the axial direction will occur.
Consequently, even during injection of a light current the forward
output light 100 will not be saturated and the single wavelength
selectivity will also be improved as in Embodiment 1.
While in Embodiments 1 and 2 the numbers of divided electrodes are
three and two, respectively, a further increase in the number of
divided electrodes will permit more complete suppression of the
hole burning. Furthermore, even if the current density distribution
of the respective laser regions for the current exceeding the
threshold current is not completely in proportion to the light
intensity distribution, the output characteristic of the laser
could appreciably be improved by simply increasing the current
density in the region near the quarter-wave phase shift point 9
(the region B in Embodiment 1) where the light intensity is
particularly high, or in the end facet portion of the laser region
30 (the region B in Embodiment 2) to a value 1.2 to 3.0 times
greater than in the other regions.
Although the present invention has been described to use
semiconductor materials of the InGaAsP/InP system, the invention is
also easily applicable to other semiconductor materials as of the
AlInGaAs/InP and AlGaAs/GaAs systems.
As described above, a DFB laser device of stable output can be
materialized through current control by the first and second
current sources 50 and 51 which are connected to each of the
divided electrodes. Moreover, according to the present invention,
the current density distribution is controlled so that during
injection of a current below the threshold current the current
density is uniform in the axial direction and that during injection
of a current above the threshold current, the density of current
exceeding the threshold current density, in the axial direction, is
high in at least the region of a maximum light intensity. This
suppresses the occurence of the spatial hole burning in the axial
direction, and consequently, it is possible to obtain a DFB laser
which is of less output saturation during the high output operation
and oscillates at a single wavelength with a high probability.
The output light of the phase shifted DFB laser can be controlled
very stable by providing two or more electrodes at both sides of
the electrode 21 overlying the region where the diffraction grating
8 has the phase shift point 9.
The high output of the phase shifted DFB laser can be stabilized by
maximizing the current density in the electrode 21 overlying the
region where the diffraction grafting 8 has the phase shift point
9.
In the DFB laser with no phase shift point a stable high output can
be obtained by dividing the electrode into two or more with respect
to the center of the active layer 3.
Besides, the present invention ensures the obtaining of a stable
high output of the DFB laser through three-step control of current
injection.
Hence, the present invention is of very wide use in the fields of
optical communication and optical data processing and is of great
utility in practical use.
* * * * *